In the exploration and production of hydrocarbons, it is desirable that the properties of the formation, such as resisitivity surrounding a wellbore, be detected. Micro-resistivity tools measure borehole surface resisitivity, and these resisitivity measurements may then be used to obtain a borehole wall image.
When measuring the micro-resistivity of a low-resisitivity formation surrounding a borehole in a non-conductive mud, the tool mandrel, the non-conductive mud and the low-resisitivity formation together form a coaxial waveguide, particularly for frequencies above about 100 kHz. The impedance that may be measured between the mandrel and the formation at the position where a measurement system is located can then be unstable. This is because any contact or partial contact between the mandrel and the formation that may occur up to tens of meters away from the measurement system can have an effect on this impedance. These variations of impedance will normally have an effect on electromagnetic measurements, particularly those above about 100 kHz, if no precautions are taken.
EP Patent Application 06292050.9 attempts to provide one solution to this problem by preventing the waveguide being excited while at the same time preventing reflections from having an effect by separating the waveguide from the measurement system by electrically decoupling the measurement system from the tool mandrel.